1. Field of the Invention
In one aspect, this invention relates to the inspection of material interfaces for the purpose of assessing the quality of the interface. In one aspect of this invention, this invention relates to material interfaces which have been joined together, such as by bonding, heat fusion, electrofusion and the like. In a further aspect, this invention relates to joined interfaces between plastic pipe sections, joined interfaces between plastic pipes and plastic fittings connected with the plastic pipe, and a method for assessing the quality of the joined interfaces.
2. Description of Related Art
Plastic pipes have been in use for many years as a means for transporting underground utilities such as natural gas and water from place to place. The benefits of using plastic pipe include corrosion-resistance, ease of installation and relatively low costs compared with metal pipes. Plastic pipe sections are typically joined together using butt fusion joints, that is, joints arising from heating the plastic pipe ends and abutting the heated ends to fuse the pipe ends together, and using electrofusion couplings. Notwithstanding the many years during which plastic pipes have been employed, integrity of the butt fusion joints remains a significant concern among users.
Currently, butt fusion joints of plastic pipes are visually inspected and a joint is rejected if it is not the proper shape. Such visual inspection works in the majority of cases; however, there have been enough joint failures that the utilities want a better inspection method. Other than careful training in the production of butt fusion joints and visual inspection, there are no acceptable non-destructive inspection methods currently available. Non-destructive methods for assessing the quality of plastic pipe butt fusion joints in the past have tended to be expensive and complex to use, rendering them generally unacceptable. One ultrasonic method of testing butt fusion joints was developed by some natural gas utilities in the 1980's as a result of which a guideline standard (ASTM-F600-78) was issued for the manual ultrasonic inspection of butt fusion joints in polyethylene pipe. However, it was withdrawn in 1991 because the results were so heavily dependent upon the skill of the operator employing the method, rendering the results necessarily inconsistent and, thus, unreliable. The UltraMc® ultrasonic inspection tool, developed and marketed by McElroy Manufacturing, Inc. of Tulsa, Okla., was not a commercial success because it too required too much operator expertise. In addition, the UltraMc tool had difficulty detecting “cold joints”. One solution that has been proposed for addressing this problem is removal of the butt fusion joint bead which is formed during the butt fusion process, polishing of the joint, and heating the joint area before applying the ultrasonic measurements. Although commercial ultrasonic equipment may be used by an expert operator to identify suspect butt fusion joints, the discrimination criteria vary significantly from operator to operator. In addition, such an approach is too expensive and, thus, rarely used.